1. Field of the Invention
The invention generally relates to the field of methods for and products of manufacturing component parts in energy storage devices and more particularly, to high surface area electrodes for supercapacitor applications.
2. Description of Related Art
In general, electrochemical capacitors are capacitive energy storage devices based on double-layer capacitance or pseudocapacitance. The potential, power density and cycle life of electrochemical capacitors are generally two orders of magnitudes higher than those of rechargeable batteries. As compared with batteries, electrochemical capacitors can be characterized as having low energy density, high power density and a high cycle life. Further, in an electric circuit, an electrochemical capacitor behaves more like a classic dielectric capacitor than a battery, hence its name.
The requirement of high energy and power density of an electrochemical capacitor intrigues development in miniaturization and weight reduction. The component parts of an electrochemical capacitor generally include at least two electrodes, electrolyte, and a separator. The material of the electrode is typically a key element. One approach to increase energy and power density of an electrochemical capacitor is to increase the accessible surface area of the electrodes. Generally, the pore size of the electrode material must be large enough to allow electrolyte access into the pores, yet small enough to provide a high surface area per volume or per weight of the electrode material. Lowering the internal resistance (e.g., resistivity of the electrode material or interface resistance between electrode constituents) of the electrode material is also a key point toward increasing conductivity and power density of electrode materials. A contact resistance between an electrode and the electrolyte and/or current collector can also increase the resistance of the capacitor.
There are four basic types of electrode materials for electrochemical capacitor applications. Activated carbon or foam represents one type of electrode material, as disclosed by U.S. Pat. No. 5,601,938. Typical capacitance obtained from an electric double layer is in the range of about 20-40 mF/cm2.
Certain transition metal oxides such as rubidium oxide (RuO2) and iridium oxide (IrO2) possess pseudocapacitance thus rendering metal oxides as a candidate for a second type of electrode material. Pseudocapacitance arises from highly reversible reactions, such as oxidation-reduction (xe2x80x9credoxxe2x80x9d) reactions, which occur at or near the electrode surfaces. Capacitance of 150-200 mF/cm2 have been observed for RuO2 films. A specific capacitance of 380 F/g has been reported using high temperature thermal treatment and 720 F/g with low temperature thermal treatment. Low temperature treatment generally forms amorphous hydra-ruthenium oxide, which tends to crystallize at temperatures above 100xc2x0 C. Ruthenium electrode material also tends to be relatively expensive. In order to reduce the cost of the expensive ruthenate electrode materials, bi-metal oxides or tri-oxides were studied, such as lead ruthenate systems having a formula A2[B2xe2x88x92x Pbx]O7xe2x88x92y, where A is lead (Pb) or bismuth (Bi); B is ruthenium (Ru) or iridium (Ir); x is greater than zero and less than or equal to one; and y is greater than zero and less than 0.5 as disclosed by U.S. Pat. No. 5,841,627.
The third type of electrode material is metallic bodies which are mechanically- or chemically-etched to provide a roughened surface and a high specific surface area, as disclosed by U.S. Pat. No. 5,062,025. High surface area metal electrodes are limited by electrochemical stability. Metals are generally unstable in oxidizing environments, therefore their use is generally limited to the positive, reducing electrode or anode.
The fourth type of electrode material is metal nitride. Metal nitrides are generally conductive and exhibit pseudocapacitance. Molybdenum nitride, for example, as pointed out at the Seventh International Seminar on xe2x80x9cDouble Layer Capacitors and Similar Energy Storage Devices, Dec. 8-10, 1997, Deerfield Beach, Fla., exhibits high energy density.
In addition to the different types of electrode materials, it has been found that the electrical performance of devices based on electrodes of consolidated powders is often limited by inter-particle electrical resistance (e.g., internal resistance), and this requires addition of conductivity-enhancing additives such as metal fibers which are themselves generally not capacitive. Consolidated powders typically have a lower powder density per unit weight of capacitor. U.S. Pat. No. 4,562,511 discusses carbon fiber electrodes. The mechanical strength of the electrode is high, and small type capacitors in various shapes are obtainable, furthermore capacitance per unit volume can be made relatively large and internal resistance and leakage current can be made relatively low.
It is desirable to provide a new type of electrode material with improved mechanical strength, reduced internal resistance and leakage current, and increased capacitance per unit volume. It is also desirable that the new type of electrode material possess high surface area and a desirable pore size.
The invention relates to a method, including a method of forming a fibrous electrode material. In one embodiment, the method includes synthesizing polymeric precursors via organic acid modification; fabricating a fibrous material of the polymeric precursors; and fabricating a body of the fibrous material.
The invention also relates to an apparatus suitable as an electrode in an energy storage device, including an electrochemical capacitor. The apparatus comprises a body having a fibrous form comprised of a moiety of the general formula:
(Ma)x(Yb)y,
wherein M is one or more metals (i.e., a is greater than or equal to one) selected from Groups IV through IX of the Periodic Table of the Elements. Examples include, but are not limited to, ruthenium, iridium, and manganese. Y includes one or more heteroatoms (i.e., b is greater than or equal to one) selected from oxygen, nitrogen, carbon, and boron. Subscripts x and y represent the valence state of the cation and anion, respectively. The invention further relates to an apparatus such as an energy storage device. In one embodiment, an energy storage device includes an electrolyte between two electrodes of fibrous material. Advantages of the device described or as formed herein in terms of electrode properties and performance compared generally to prior art devices include: (1) reduced internal resistance and leakage current of the supercapacitive device, therefore improving power density; (2) increased specific surface area of fibrous electrode material, therefore higher energy density; (3) enhanced mechanical strength of the electrode, therefore lowering the contact resistance.
Additional features, embodiments, and benefits will be evident in view of the figures and detailed description presented herein.